The recently described mouse cell lines HCD-9 and SC-10 are derived from the Friend Murine Leukemia Virus. These cells absolutely require added erythropoietin for growth in culture. These are the first sucn cell lines described. We have further characterized these cell lines and demonstrated erythroid differentiation in response to hemin: morphologic maturation, benzidine stain positivity, and increased globin RNA content. Further studies are focusing on the erythropoietin receptor in these cells. We have also studied the mouse cell lines IW-32 and TP-3. These cells abnormally secrete erythropoietin. We have cloned the erythropoietin gene and flanking DNA regions from each cell line. Thus far, IW-32 shows a rearrangement 5' to the gene by restriction mapping. We are sequencing both this gene and its normal counterpart. Future studies will assess the functional significance of this rearrangement. We will also look for mutations affecting control of the gene in TP-3.